1. Field of the Invention
The present invention relates to improvements in availability of network communication in a client-server system which is connectable to an external network, such as the Internet.
2. Description of the Related Art
FIG. 13 shows a basic structure of a network system constructed as a client-server system in which access to the Internet is enabled. FIG. 13 shows a structure wherein a gateway (GW) 4 connected to the Internet 1 via a communication channel 3, a DHCP (Dynamic Host Configuration Protocol) server 5, and one or a plurality of clients 6 which access the Internet 1 are connected to a LAN (Local Area Network) 7.
With this structure, when a client 6 accesses the Internet 1, the client 6 transmits an IP address assignment request to the DHCP server 5. In response to the IP address assignment request, the DHCP server 5 assigns an IP address to the client 6 and returns the assigned IP address with the known address of the gateway 4. Based on the information sent from the DHCP server 5, the client 6 identifies a gateway 2 to which the client 6 requests connection to the Internet 1 and accesses the Internet 1 via the gateway 4 and the communication channel 3 using the IP address assigned by the DHCP server 5.
There is a general trend in recent years that the number of clients 6 connected to one LAN 7 is increasing. As the number of connected clients 6 increases, the load placed on each device also increases. In order to handle the increased load, a network administrator may attempt to, for example, strengthen the communication capability by, for example, employing a lease line as the communication channel 3, improving the processing capability of the server 5 and/or gateway 4, or setting various proxy servers. Alternatively, the network administrator may divide the LAN 7 into segments and provide a communication channel 3, gateway 4, and server 5 as shown in FIG. 13 separately for each segment of the divided LAN.
The process for the client 6 to obtain an IP address and address of the gateway 4 will now be described for a system structure wherein a plurality of gateways 4 and DHCP servers 5 are connected to a LAN 7.
A client 6 which has not yet obtained an IP address broadcasts a DHCPDISCOVER message. All DHCP servers 5 receive this message, and, among the DHCP servers 5, one or more DHCP servers 5 which are programmed to respond to the client 6 return a DHCPOFFER message to the client 6. The DHCPOFFER message includes information such as the IP address to be assigned, the address of the gateway 4, and the lease period of the IP address. The client 6 collects the response DHCPOFFER messages from the DHCP servers 5 and selects one of the DHCPOFFER messages. The client 6 then transmits a DHCPREQUEST message to the corresponding DHCP server 5. In response to the received DHCPREQUEST message, the DHCP server 5 responds by transmitting a DHCPACK so that lease of the IP address can be started. In this manner, the client 6 can access the Internet 1 using the assigned IP address and gateway 4.
In DHCP, a lease period for an IP address is set by the DHCP server 5 and the IP address cannot be used beyond the expiration of the lease period. Because the DHCP server 5 has the authority to control the lease, a client 6 which wishes to continue accessing the Internet 1 requests extension of lease to the leasing DHCP server by transmitting a DHCPREQUEST message. If the DHCP server 5 accepts continuation of use, the DHCP server 5 transmits a DHCPACK in response to the received DHCPREQUEST message. In this manner, the client 6 can continue to use the assigned IP address and gateway 4 to access the Internet 1.
If, on the other hand, the DHCP server 5 rejects the continued use of the IP address, the DHCP server 5 transmits a DHCPNACK. When the client 6 receives the DHCPNACK, because the client 6 cannot use, after the lease period is expired, the IP address which is being used, the client 6 returns to its initial state, that is, the state before the IP address is obtained. Therefore, the client 6 again broadcasts the DHCPDISCOVER message as described above and obtains an IP address.
When there is no response from the leasing DHCP for the transmitted DHCPREQUEST, the client 6 assumes that the DHCP server 5 is either suspended or in a unreachable state, and broadcasts a DHCPREQUEST message. In this manner, the client 6 obtains an IP address and an address of the gateway from another DHCP server 5.
However, in the conventional art, because the minimum length of the lease period which is set by the DHCP server is set at 1 hour, a length of 1 hour or more is set for all clients and a uniform lease period is set regardless of the gateway to be used. Because of this, although when a certain gateway is suspended by a failure occurring in the gateway, another available gateway connects the client and the Internet in place of the suspended gateway, even when the suspended gateway is restored, the client cannot change the gateway to be used back to the original gateway. That is, when, for example, the DHCP server uniformly distributes the clients to each gateway in consideration of the load distribution, in a case wherein a certain gateway is suspended and is later restored, the load is concentrated to the substituting gateway as long as the client continues to use the IP address, even when the suspended gateway is restored.